The vacuum supply for the vacuum brake booster is normally executed by means of a driving engine (internal combustion engine). If the maximum attainable boosting force is insufficient due to an only weak vacuum supply, which becomes more frequent in novel engine technology such as a direct gasoline injection system or Diesel engine, there is need for an additional brake boost.
One possibility of producing an additional brake force or an additional brake pressure involves the use of ‘active’ hydraulic brake boost. This is achieved e.g. by means of a hydraulic pump. The hydraulic pressure, which results in the hydraulic master brake cylinder from the brake force that is introduced by the driver by way of the brake pedal and boosted by means of a vacuum brake booster is additionally increased by the hydraulic pump. Said pump is driven by an electric motor actuated by an electronic brake control unit. An actuatable separating valve is arranged in a connecting line between the master brake cylinder and the wheel brakes for the purpose of regulating or controlling the brake pressure.
It is known in the art to configure these valves as analog or analogized valves, allowing the adjustment of intermediate positions in addition to an open and a closed switch position. It is thus possible to adjust a defined volume flow through these valves.
It is necessary in systems of this type to know about the valve characteristic curves of the separating valves because pedal comfort depends on this knowledge. If a stored characteristic curve is not in conformity with the actual characteristic curve, an excessively large valve opening may be produced in the transition from the closed valve to the partly open (analog) valve. From this results a too large volume flow through the valve what the driver can feel at the brake pedal. If a volume flow in the direction of the master brake cylinder is very large, a pressure rise in the master brake cylinder can even cause a wrong interpretation of the driver's request. Pressure build-up may then be unduly initiated. The driver registers this condition, too. He/she senses that the pedal moves away or is ‘sucked away’ due to the erroneously triggered pressure build-up. This situation can also occur several times during release of the brake pedal. The negative effect is particularly easy to recognize in these vibrating modulations because a first error control will also cause a subsequent error control.